The present invention relates to computer animation, and more particularly to pre-analysis of video signals obtained from digital images produced from the computer animation process to assure that the digital images will produce video signals compatible with video broadcast standards.
In computer animation for the television industry an artist generally works with a computer workstation in the red, green and blue (RGB) domain. The workstation display is in the form of a non-interlaced bit-map. Each "bit" in the map represents a picture element (pixel), and for RGB is commonly a 24-bit digital word, eight bits for each color component. Thus the range of values is from zero to 255 for each component. Because the resulting image is digital, there can be sharp transitions between color levels. A sequence of these images may be stored in a computer file as an animation sequence.
Alternatively the artist may work in a digital component domain where each pixel of the image is described in terms of a luminance component Y (brightness) and two chrominance components Cr, Cb. Again due to the digital nature of the signal there may be sharp transitions between colors in the image. Also it can be shown that there are combinations of values for Y,Cr,Cb that produce colors which are not reproducible in the RGB domain. So transformation from one color domain to another may result in invalid colors in the target color domain.
In the digital component domain the luminance Y is defined as percentages of the RGB components. For example: EQU Y=0.299R+0.587G+0.114B.
Each chrominance component is then defined as a function of R-Y (Cr) and B-Y (Cb). For broadcast applications this digital component video is then converted into a composite signal where one color component modulates a color subcarrier signal, the other color component modulates a quadrature version of the color subcarrier signal, and the two modulated signals are added together. To this composite chrominance signal is added the luminance signal. Because of limitations in bandwidth imposed by the broadcast television standards, the sharp transitions in color or luminance may cause ringing in the composite video monitors or video receivers which produces objectionable "ghosting" in the image. Since the audio portion of the broadcast signal is frequency modulated against the video portion, if the luminance and chrominance components go too high, the AM portion of the signal goes to zero, resulting in buzz or other audio effects. Also where textures in the image vary at frequencies close to the subcarrier frequency, the video monitor/receiver is confused into interpreting luminance information as chrominance information, producing an erroneous color display--a "rainbow" effect.
The artist produces the animation sequence and generally provides it to a broadcast station or post-production studio in a digital component video format. When this digital component video animation sequence is encoded into a broadcast video signal, the above problems may appear for the first time. This is either too late to change the animation sequence, or will require an extensive amount of time by the artist to correct the animation sequence in order to avoid the distortions.
What is desired is a pre-analysis of video signals obtained from a digital image that provides an artist in the preparation of animation sequences with an indication of color distortion errors so that the artist can correct such errors interactively during the animation process.